1. Field of the Invention
The present invention relates to one-component polyurethane stoving coating compositions containing polyisocyanate crosslinking agents blocked with CH-acidic esters, especially malonic acid esters, and to their use for coating plastic parts.
2. Description of the Prior Art
The blocking of polyisocyanates with CH-acidic esters, especially malonic acid esters is known and described by the following publications: Petersen, Liebigs Ann. Chem. 562, (1949) 205, DE-A 30 010 60.
Using polyisocyanates blocked with malonic esters, and polymeric OH components (polyesters, polyacrylates, etc.), it is possible to prepare storage-stable “one-component coating compositions”. After application, the coating compositions cure under stoving conditions of 30 minutes at 100° C. With other isocyanate-blocking agents, for example butanoneoxime, 1,2,4-triazole, diisopropylamine or 3,5-dimethylpyrazole, such a low stoving temperature is not achievable.
However, a low stoving temperature is necessary when thermoplastic plastics, for example motor vehicle parts such as bumpers, hub caps or trims, are to be coated. Recently, the challenge has been to coat polypropylene parts with a one-component coating composition, which requires stoving temperature of less than 100° C.
It is known to process one-component polyurethane stoving lacquers with the conventional blocking agents mentioned above with, for example, dibutyltin dilaurate (DBTL) catalysts (e.g. in Farbe&Lack 7/96, 102nd year, pages 51-58, Vincentz Verlag/Hanover).
Catalysis of one-component stoving lacquers containing polyisocyanates blocked with malonic esters was previously not known.
Accordingly, an object of the present invention is to cure one-component polyurethane stoving lacquers based on polyisocyanates blocked with CH-acidic esters at stoving temperatures of <100° C.
It has been possible to achieve that object using the one-component polyurethane stoving lacquers according to the invention, which contain titanium compounds as catalysts.